Gamma amino butyric acid (GABA) levels in hypophyseal stalk plasma of rats.

To determine the possible physiologic contribution of GABA to the tonic hypothalamic inhibition of adenohypophyseal prolactin secretion, we compared GABA levels in hypophyseal stalk plasma with those found in the peripheral circulation. Hypophyseal stalk blood was collected via a parapharyngeal approach from 8 urethane anesthetized diestrus rats. Peripheral blood was collected simultaneously from the external jugular vein of the same rat at a rate similar to hypophyseal stalk blood flow. Blood samples resulting from a single 4 hr collection per animal were centrifuged, and the plasma stored frozen before ethanol extraction and assay using a radioreceptor method. GABA levels in hypophyseal stalk plasma (909 +/- 171 pmol/ml; X +/- S.E.M.) were not significantly higher than levels in peripheral plasma (845 +/- 182; p greater than 0.05), indicating little or no secretion of GABA by the median eminence.